[ObjC ARC] Converts ios/chrome/browser/ui/reader_mode:reader_mode to ARC.

ios/chrome/browser/ui/reader_mode/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 source_set("reader_mode") {
+  configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
     "reader_mode_checker.h",
     "reader_mode_checker.mm",
     "reader_mode_controller.h",
     "reader_mode_controller.mm",
     "reader_mode_infobar_delegate.h",
     "reader_mode_infobar_delegate.mm",
     "reader_mode_view.h",
     "reader_mode_view.mm",
   ]
   deps = [
     "//base",
     "//components/dom_distiller/core",
     "//components/infobars/core",
     "//components/resources",
     "//ios/chrome/browser/browser_state",
     "//ios/chrome/browser/dom_distiller",
     "//ios/chrome/browser/infobars",
     "//ios/chrome/browser/ui/material_components",
     "//ios/third_party/material_components_ios",
     "//ios/web",
     "//ui/base",
     "//url",
   ]
   libs = [ "UIKit.framework" ]
 }